(1) Field of the Invention
The present invention lies within the field of shaping frames made of laminated composite materials in order to form structural elements of an aircraft fuselage. The present invention relates more particularly to the layout of a zone of reinforcement of such a frame that is shaped into a curve and that interconnects two zones of the frame to be joined together, each such zone to be joined together having a respective orientation.
In the field of aviation, aircraft fuselages are typically arranged as a longitudinally elongate shell. Such a shell is primarily made as a skin covering a framework. The framework is made up in particular of load-bearing members or skin-stiffening members that are arranged and fastened together in order to confer a desired shape to the shell. Among such stiffening members, some are shaped as frames that may either be open or closed. The frames may potentially be of any shape, being defined depending on shape requirements for the framework.
(2) Description of Related Art
Such a frame is commonly made from a profile that is shaped to confer a desired shape to the frame. The profile presents a cross section of any shape, which cross section is determined depending on the strength and stiffness desired for the frame given the forces to which it is subjected. Such a profile commonly comprises a core of elongate cross section that is suitable for being provided with a flange at at least one of its ends. The profile may also comprise two flanges disposed at either end of the core, i.e. an inside flange facing towards the inside recess of the frame and an outside flange facing towards the outside of the frame. Such a configuration for the cross section of a profile having two flanges may provide a cross section in the shape of the letter C, I, J, or Ω, for example.
The profile may be built up from laminated composite materials, having various layers (or “strates” [plies] in the terminology of the “Dictionnaire Encyclopédique des Matériaux Composites” [Encyclopedic Dictionary of Composite Materials], WEKA INDUSTRIE, 1985, updated 1988) that are made of a stack of superposed fabrics or sheets. The fibers of the sheets extend in directions that depend on specific strength and stiffness requirements of the frame. The various layers of composite materials conventionally associate sheets having fibers that are unidirectional or almost-unidirectional with sheets having fibers that are bidirectional, then referred to as “woven fabrics”.
Conventionally, a sheet is made up of unidirectional fibers, i.e. fibers all extending in a single direction. Such a sheet is optionally packaged as a roll or reel that makes it possible to use it in a filament depositing machine. Woven fabrics are packaged in rolls and include fibers extending in the warp direction of the fabric and fibers extending in the weft direction of the fabric. For a sheet with almost all its fibers unidirectional, optionally referred to as “unidirectional woven fabric”, almost all of the fibers extend in the warp direction of the fabric, with the remainder of the fibers extending in the weft direction of the fabric. For woven fabric with bidirectional fibers, the fibers are generally distributed substantially equally between the warp direction and the weft direction of the fabric.
Typically, bending forces on the frame generate in the profile both shear stresses and perpendicular angle stresses extending in the long direction of the profile. Conventionally, the perpendicular angle stresses are taken up by the flanges, e.g. by unidirectional fibers, and shear stress is taken up by the core, e.g. by bidirectional fabrics with fibers that then extend mainly at ±45°. Naturally, fabrics having bidirectional fibers may also be incorporated in the flanges in addition to the unidirectional fibers.
In this context, the frame may include curved zones interconnecting zones of the frame to be joined together, which said zones to be joined together are located at respective ends of a given curved zone. Typically, the various curved zones of the frame interconnect the zones to be joined together, and each of them is geometrically defined by a radius of curvature identified by a value and by a center of curvature in a position that is constant relative to the frame. For illustrative purposes, a said curved zone is commonly located in a corner of the frame in order to interconnect zones to be joined together that extend in straight lines.
By way of example, reference is made to US documents 2010/136293 (KUBRYK VANESSA et al.), EP 0 346 210 (AEROSPATIALE), US 2009/202763 (ROSE DONALD et al.), and US 2008/179460 (RODRIGUEZ ELENA AREVALVO et al.) that describe such a frame used to form the fuselage of an aircraft.
However, when the perpendicular angle stresses to which the profile is subjected in the curved zone of the frame are in equilibrium, that generates stresses in the flanges that extend perpendicularly to the planes of the flanges. It has been observed that such stresses, referred to as delamination stresses, are proportional to said perpendicular angle stresses divided by the value of the radius of curvature of the curved zone under consideration of the frame, and they tend to cause delamination of the profile, in particular in the junction zones between the core and the flanges.
In theory, the delamination stresses are taken up by adhesively bonding the sheets to one another. However, the ability of composite materials to withstand delamination stresses is low, because the various sheets constituting the composite materials forming the profile usually have no fibers that extend along the direction of the delamination stresses. As a result, the delamination stresses act on the matrix of the composite material, the strength of said matrix being significantly lower than that of the fibers.
This shows that it is necessary to reinforce the curved zones of the frame in order to avoid delamination of the profile, and more particularly delamination of an inside flange of the profile, whenever the bending moment that is applied to said profile and that is of axis perpendicular to the core tends to put the inside flange of the frame into traction.
Among the current solutions aiming to reinforce the curved zones of a composite material frame, it is possible to select a value for the radius of curvature of a curved zone to be considerable, or for the flanges of the profile to be overdimensioned.
Other known solutions for avoiding separation of the sheets in the event of delamination of the profile consist in using rivets or in assembling the various sheets of the flanges together by stitching, or even in placing reinforcing gussets in the curved zones.
Although expensive and difficult to implement while fabricating the profile, another possible solution consists in using sheets of fabric having multidirectional fibers. However, in that solution, it is necessary to use three-dimensional weaving to make the profile, or at least its flange.
For information about a technological environment that is close to the present invention, reference may be made to document WO2009/112694 (AIRBUS OPERATION SAS) that describes an embodiment of a structural frame of an aircraft fuselage and a method of fabricating such a frame.